The present invention relates to a method for producing molecular sieves.
Molecular sieves are a commercially important class of crystalline materials. They have distinct crystal structures with ordered pore structures which are demonstrated by distinct X-ray diffraction patterns. The crystal structure defines cavities and pores which are characteristic of the different species. Natural and synthetic crystalline molecular sieves are useful as catalysts and adsorbents. The adsorptive and catalytic properties of each molecular sieve are determined in part by the dimensions of its pores and cavities. Thus, the utility of a particular molecular sieve in a particular application depends at least partly on its crystal structure. Because of their unique sieving characteristics, as well as their catalytic properties, molecular sieves are especially useful in such applications as gas drying and separation and hydrocarbon conversion. The term xe2x80x9cmolecular sievexe2x80x9d as used herein refers to a material having a fixed, open-network crystalline structure, that may be used to separate hydrocarbons or other mixtures by selective occlusion of one or more of the constituents, or may be used as a catalyst in a catalytic conversion process.
Zeolitic Molecular Sieves
One type of molecular sieve is a crystalline zeolite. Crystalline zeolites may be divided into two general types based on crystal structure considerations. One type includes zeolites having a SiO2/Al2 O3 molar ratio in the crystalline lattice typically less than 12, which are conventionally prepared without an organic templating agent. Many of these zeolites also contain sodalite substructures, and have a tetrahedral atom density of less than about 15 TO2/1000 xc3x853. Zeolites having these general characteristics include, for example, zeolites A, N-A, ZK-4, faujasite, X, Y, ZK-5 and.
A number of processes have been offered for preparing crystalline zeolites of this type within discrete particles. For example, Howell, et al., in U.S. Pat. No. 3,119,660 teaches a method for producing crystalline metal aluminosilicate zeolite by reacting preformed bodies of clay particles in an aqueous reactant mixture including alkali metal oxide. Similar processes for preparing zeolites from formed bodies, which may contain zeolitic seed crystals, in alkali solutions are also taught in U.S. Pat. No. 4,424,144 to Pryor, et al., U.S. Pat. No. 4,235,753 to Brown, et al., U.S. Pat. No. 3,777,006 to Rundell et al., U.S. Pat. No. 3,119,659 to Taggart et al, U.S. Pat. No. 3,773,690 to Heinze et al., U.S. Pat. No. 4,977,120 to Sakurada et al and GB 2 160 517 A. U.S. Pat. No. 3,094,383 teaches a method of forming an A type zeolite by aging a homogeneous reaction mixture out of contact with an external aqueous liquid phase but under conditions to prevent the dehydration of the mixture. GB 1 567 856 discloses a method of preparing zeolite A by heating an extruded mixture of metakaolin powder and sodium hydroxide.
In U.S. Pat. No. 4,058,586, Chi et al. discloses a method for crystallizing zeolites within formed particles containing added powdered zeolite, where the formed particles furnish all of the liquid needed for crystallization. Crystallizing the particles in an aqueous alkaline solution is not required using the process of Chi et al.
Verduijn, in WO 92/12928, teaches a method of preparing binder-free zeolite aggregates by aging silica-bound extruded zeolites in an aqueous ionic solution containing hydroxy ions. According to the disclosure of Verduijn, the presence of zeolite crystals in the extrudate is critical for making strong crystalline zeolite extrudates. Verduijn, et al., in EPO A1/0,284,206, describe a method of preparing binderless zeolite L by forming silica and preferably 10-50 wt % preformed zeolite L crystallites into particles, and then reacting the particles with an alkaline solution containing a source of alumina to form the zeolite L. More recently, similar methods have been proposed for preparing high silica zeolitic materials. Conventional methods for preparing high silica materials, having a SiO2/Al2O3 molar ratio of greater than about 10, and more typically greater than about 20, typically involves crystallizing the zeolites from aqueous solution. For example, U.S. Pat. No. 3,702,886 to Argauer, et al., teaches a method of preparing ZSM-5 from a solution containing tetrapropyl ammonium hydroxide, sodium oxide, an oxide of aluminum or gallium, an oxide of silica or germanium, and water. The digestion of the gel particles is carried out until crystals form. The crystals are separated from the liquid and recovered.
A variation of the preparation procedure involves using clay as a source of alumina in preparing high silica zeolites. For example, U.S. Pat. No. 4,091,007 discloses a method for preparing a crystalline aluminosilicate zeolite, specifically ZSM-4 or ZSM-5, from a reaction mixture where at least about 70 weight percent of the alumina is provided by an alumina-containing clay added to the reaction mixture. EPO A2/0,156,595, discloses the preparation of crystalline zeolites having a silica to alumina mole ratio greater than 12 and a Constraint Index of 1 to 12 by forming a mixture of seed crystals, a source of silica, a source of alumina and water into shaped particles, which are then crystallized in an aqueous reaction mixture containing a source of alkali cations. It is also taught that alumina-containing clay may be used as an alumina source. U.S. Pat. No. 4,522,705 is directed to a catalytic cracking catalyst comprising an additive prepared by the in-situ crystallization of a clay aggregate disclosed in EPO A2/0,156,595. U.S. Pat. No. 5,145,659 teaches methods for increasing the silica content of a zeolite supported on a matrix, where the matrix may be a clay.
Special methods for preparing the reaction mixture from which a zeolite may be crystallized have also been proposed. In U.S. Pat. No. 4,560,542 a dried hydrogel containing silica and alumina is contacted with a fluid medium containing an organic templating agent and maintained at specified crystallization conditions to form a crystalline aluminosilicate.
In U.S. Pat. No. 5,240,892 a reaction mixture containing at least about 30 weight percent solids content of alumina and precipitated silica is taught for preparing zeolites. The method of preparing the reaction mixture allows agitation of the mixture during crystallization, in spite of the high solids content of the mixture.
Zeolite crystallization from reaction mixtures initially containing a gel-like phase in equilibrium with an excess of liquid phase is disclosed in R. Aiello, et al., xe2x80x9cZeolite Crystallization from Dense Systemsxe2x80x9d, Materials Engineering 1992, Vol. 3, n. 3, pp.407-416.
The traditional conventional method of manufacturing molecular sieves involves providing the reaction mixture with a sufficient amount of water to cause the crystallization to begin in the presence of an added external water phase. Such a high-water mixture is prepared in a gel tank and pumped into an autoclave reactor which may comprise a double-walled vessel through which hot oil is conducted to heat the mixture. The reactor includes a mechanical paddle-type of stirrer which stirs the mixture to distribute the heat. As noted above, crystallization occurs in the presence of a separate or external water phase, requiring that special measures be taken to separate water from the molecular sieves. Also, the separated water may have to be treated for environmental reasons.
More recently, methods have been devised for the manufacture of molecular sieves which avoids problems associated with the above-disclosed high-water methods. In that regard, low-water methods have been described, for example, in ZEOLITES, 1992, Vol 12, April/May, p. 343; ZEOLITES 1990, vol 10, November/December, p. 753; ZEOLITES 1989, vol 9, November, p. 468; Nature, Vol 317(12), September 1985, p. 157; and J. Chem. Soc., Chem. Commun., 1988, p. 1486. J. chem. Soc., Chem. Commun., 1993, p. 659 describes a kneading method for synthesizing ZSM-35 in a nonaqueous system, in which the amount of liquids used to prepare a crystallization mixture is not sufficient to wet all the solid particles so that the agglomerate reactant is actually a mixture of dry powder and small doughy lumps. The method can thus be considered to constitute a low water or in-extrudate method in that crystallization occurs in the absence of an external water phase.
A particularly advantageous low-water, or in-extrudate, process for preparing a crystalline zeolite has been discovered by one of the present inventors and disclosed in U.S. Pat. Ser. No. 5,558,851, the disclosure of which is incorporated by reference herein. For example, that patent discloses to prepare a reaction mixture which comprises at least one active source of a first oxide selected from the group consisting of an oxide of silicon, germanium or both, optionally at least one active source of a second oxide selected from the group consisting of an oxide of aluminum, boron, gallium, iron or a mixture thereof, an organic templating agent capable of forming the crystalline zeolite, and sufficient liquid, e.g., water, to shape the mixture. The mixture is heated at crystallization conditions and in the absence of an added external liquid phase for sufficient time to form a crystallized material containing crystals of the zeolite.
If it is desired to form the crystals from shaped or formed particles, it may be desirable to pre-dry the reaction mixture to remove water, leaving enough water to shape the mixture. That is, once the reaction mixture is formed into the desired shape, containing the desired amount of water, the resulting shape is self-supporting.
U.S. Pat. No. 5,785,945 (the disclosure of which is incorporated herein by reference), an in-extrudate method of preparing crystalline zeolite L is described. That method comprises preparing a reaction mixture comprising at least one active source of silica and at least one active source alumina in amounts sufficient to produce zeolite L, and sufficient water to produce zeolite L, and heating the reaction mixture at a temperature from about 100xc2x0 C. to about 200xc2x0 C. under crystallization conditions and in the absence of an added external liquid phase for sufficient time to form crystals of zeolite L.
An in-extrudate method for preparing crystalline Y zeolite is described in U.S. Pat. No. 5,785,944, the disclosure of which is incorporated by reference herein. That method comprises preparing a reaction mixture comprising at least one active, non-zeolitic source of silica and at least one active, non-zeolitic source of alumina in amounts sufficient to produce Y zeolite, and sufficient water to produce Y zeolite, and maintaining the reaction mixture at a temperature of up to about 130xc2x0 C. under crystallization conditions and in the absence of an added external liquid phase for sufficient time to form crystals of Y zeolite.
Non-Zeolitic Molecular Sieves
In addition to zeolitic molecular sieves, there are also known non-zeolitic molecular sieves, such as aluminophosphates. Conventional high-water methods for preparing aluminophosphate-containing molecular sieves are taught, for example, in U.S. Pat. Nos. 4,310,440; 4,440,871; 4,567,029; 4,686,093; 4,913,799 and 4,973,785. An advantageous in-extrudate (low-water) method for producing non-zeolitic molecular sieves is disclosed in U.S. Pat. No. 5,514,362, the disclosure of which is incorporated by reference herein. In that method, non-zeolitic molecular sieves are prepared from a reaction mixture comprising self-supporting particles wherein the particles comprise active sources of the molecular sieve. More specifically, the method involves producing particles comprising at least one active source of phosphorous, at least one active source of alumina, an organic templating agent capable of forming the molecular sieve and sufficient water to shape the particles. The particles are maintained at crystallization conditions for sufficient time to form a crystallized product comprising crystals of the molecular sieve.
Due to the relatively low quantity of water in the reaction mixture used in the low-extrudate methods of making zeolite or non-zeolite molecular sieves, it is not possible to use a conventional mechanical stirrer since the stirrer would encounter too much resistance in the high-solids mixture (in contrast to the low resistance encountered in the low-solids mixture used in the conventional methods). For the same reason, if a low water method using a reaction mixture comprised of formed particles were being used, a conventional mechanical stirrer would impart considerable shearing forces to the formed particles which could break those particles. On the other hand, in the absence of a thorough mixing of the reaction mixture during the heating phase, the reaction mixture will not be uniformly heated; and if formed particles are present in the reaction mixture, clumping of those particles could occur.
Those problems might be alleviated by using a smaller quantity of reaction mixture in the reactor, but then the rate of production would be undesirably reduced.
Accordingly, it is an object of the present invention to provide a novel method for preparing molecular sieves.
Another, object is to provide such a method which enables molecular sieves to be produced from either powder or formed particles.
A further object is to provide such a method which can be used in the presence of either high or low water levels.
Another object is to provide such a method which uses a low water volume while achieving a highly uniform and rapid distribution of heat to a reaction mixture, without breakage or clumping of shaped particles.
Still a further object is to provide such a method which maximizes the amount of molecular sieve per unit volume of product.
Still another object is to provide such a method which involves the production of either zeolitic or non-zeolitic molecular sieves.
A method has been discovered which surprisingly enables an in-extrudate reaction mixture to be effectively crystallized to produce zeolitic or non-zeolitic molecular sieves. The reaction mixture is heated within a slowly rotating, double-walled reactor vessel wherein a heated medium is conducted within a space formed between the double walls of the vessel. Thus, the reaction mixture is contacted by a uniformly heated wall while being gently tumbled at low speed. The tumbling action serves to uniformly distribute the heat within the reaction mixture without subjecting the mixture to such shearing that could damage shaped particles. A relatively high quantity of reaction mixture can be handled in that way to maximize the production volumes and reduce production costs.
Moreover, the method is applicable to conventional high-water methods as well as to low-water methods of making either zeolitic or non-zeolitic molecular sieves.